18 year old Weil McLain boiler not working anymore?

Discussion in 'Boiler Forum' started by Peze, Jan 28, 2012.

  1. Peze

    Peze New Member

    Messages:
    4
    Location:
    New Jersey
    I have a model CG 6-SPDN boiler built in 1994 for my hot water radiant heat (radiators) and will be using it also for radiant floor heating in an addition I am building. It has been running fine without any issues, however, a few weeks ago, it started acting differently.

    The boiler seems to try to maintain the "system" temperature all the time, even if no zones are calling for heat. I even turned off the thermostats - and even disconnected the wires from the zone control to the boiler. Once the water temperature drops below 160F, it starts heating. It then heats until the temperature reaches about 200F and then turns off. The temperature limiter in the boiler (the little wheel) is set to 170F. The only way to stop this is to turn off the power to the boiler.

    The next issue is that the boiler temperature is dropping by maybe 1-2 degrees a minute, so it starts firing up again about every 30 minutes. Is that normal that the temperature drops that quickly? is there some insulation missing or can I add insulation somewhere?
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,184
    Location:
    Maine
    It's got a leak somewhere. The temperature drops because its getting constantly filled with cold water. Shut the feed to it off and see if the pressure drops
  3. Peze

    Peze New Member

    Messages:
    4
    Location:
    New Jersey
    the cold water feed to the heating system is always turned off, while the heating system is in operation. However, I found that the flow control valve was in the open position and not set to actually prevent the gravity flow. I set it properly going that this could have done it. I did notice that the radiators now are not always hour anymore, so tours a goid thing. The boiler is still constantly firing up without any zone calling for heat and the temperature is ask dropping. The pressure is only at about 5psi, but it doesn't change our get less. What should the pressure be?

    I didn't find a leak in any of the piping. Did you mean that the could would have a leak our the piping?
  4. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,184
    Location:
    Maine
    Pressure should be between 12 and 18 lbs normally.
    You have a dual aquastat on the boiler.
    High limit shuts it off and the low turns it on.
    If the low limit is set at 160, the boiler will maintain at least 160 all the time regardless of what the thermostats are set at.
    The differential should be set at at 15 or 20 to avoid short cycling.
    If you are not using the boiler to make domestic hot water then crank the low limit all the way down or by-pass it by removing and capping the blue wire if it is a Honeywell L8124
  5. Peze

    Peze New Member

    Messages:
    4
    Location:
    New Jersey
    Thank you for your info so far. First thing I did was filling in more water to raise the pressure. The boiler now shows consistently a pressure of about 15 psi. I also adjusted the flow control valve as I mentioned before. The first issue I did have was that the radiators stayed completely hot, even though the room temperature hit the set temperature on the t'stat and the circulator turned off. But I guess the open flow control valve let the hot water still raise through the pipes to the radiators and kept heating them. I guess that also explains some of the quick temperature drop. It seems like the boiler is firing up less often now and keeps the temperature longer. I was not able to measure the difference yet, since I didn't have time to just sit there and wait for the boiler to fire up.

    One thing that i still am curious about and am wondering about: As I stated before, the boiler fires up once the temperature goes below about 160 degrees and heats up to about 200 degrees. I looked at the different controls and did not find a way to adjust the low limit setting. I have a relay "White-Rodgers 631-9001X" and a thermostat/limit control from Honeywell and the only markings on there I could decipher said "MPLS" and "198196". That control has a little wheel ranging from 140 - 240 degrees. In the wiring diagram on the boiler it is just mentioned as "additional limit control". Could that actually be the low limit control? Since it would make sense that the boiler starts heating once it goes below 160 degrees if I set it to 170 and consider it a low limit control. It would also explain, why the temperature goes up to 200+ degrees and doesn't stop at 170.

    I mean for now the boiler at least has enough pressure and doesn't cool off that quickly and I can control the temperature. Now the only problem that is left is that I would like to adjust it to work more efficiently and not necessarily keep the water temperature always at this high temp and hopefully can also have it stop heating that often.

    Is there any way to add additional insulation to it?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,055
    Location:
    New England
    Insulating the boiler probably isn't feasible, but insulating the pipes is.

    An ideal situation on the newer boilers that can handle the lower temps is to run constantly - that way, the comfort level is improved (no or limited cycling) and lower water temps are more efficient. But, older boilers can be damaged with lower temps (return less than 130 or so) as that can create condensation which can corrode things and leads to early failure. how to optimize what you have would take more knowledge of what you have, how it is configured, and what your loads actually are.
  7. Dana

    Dana In the trades

    Messages:
    2,864
    Location:
    01609
    According to the manual, the boiler's temperature controls have a fixed ~30F hysteresis- and calls for heat only control the circulator pump. So with the aquastat set to 170F it SHOULD fire until it hits ~200F, turn off, then re-fire when it drops below 170F. On a call for heat from a low temp zone or a zone that's been off for awhile it's likely that it can drop substantially below 170F, so 160F isn't a surprise. Dropping to 160F would be more likely during milder weather than during a cold snap, since the time between calls for heat are longer. It sounds as if it's behaving normally in that regard.

    If the system has enough heat emitters to deliver enough heat to keep up even at a lower boiler temp even during the colder weather, if the boiler is in an unheated basement you can save 2-3% on fuel for every 10F you drop the boiler temperature. Since the aquastat can adjust as low as 140F, give it a shot, and bump it up 5-10F at a time if the zones don't keep up on the extremely cold days. Since you have evidence that it can undershoot the aquastat setting by 10F going lower than that wouldn't be a good idea even if it can be turned down further. As jadnashua correctly points out, cast iron boiler can suffer condensation damage on the heat exchangers if they regularly fire with 130For lower return water, but it'll be fine to let the boiler hit those temps briefly on a burn where it goes relatively quickly back up to (140F + 30F=) 170F, re-evaporating any condensation that formed at the beginning of the burn. This is very different from being fed sub-130F return water continuously, potentially accumulating condensation on the heat exchangers throughout the burn. Its even less of an issue if it's plumbed with a system-bypass/boiler-bypass piping, mixing some of the boiler output with the return water, as long as the boiler fires up promptly at or above 130F.

    Lowering the boiler temp 20-30F lowers the standby loss considerably. In an uninsulated basement that standby loss it almost all a true loss, and not supporting the heat load of the whole house. Lowering the average output temp 20-30F also lowers the losses from the distribution plumbing, but it's still worth insulating any of the heating system plumbing (even the returns).
  8. Peze

    Peze New Member

    Messages:
    4
    Location:
    New Jersey
    Thank you for all your info! The idea of insulating the pipes is greatly appreciated, since I did not think of that yet and it probably will make quite a difference.

    @Dana:
    Thank you also for pointing out to look for the manual. I am wondering if the manual you referenced is for the current boilers and if they behave differently than the one I have. I looked for the discontinued boilers at Weil McLain's website and found the following manual, which seems to be the old model I have. On page 7 it references the boiler water temperature and it references the high limit control. So it seems that this control is really a high limit control and then I would assume that the control is either shot or that the sensor is not ok or something like that.

    I guess I am going to have a pro come and look at it - and once it works properly - i can hopefully insulate the piping and lower the temperature and maybe install a system bypass!?

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